This invention relates to vacuum-pressure swing adsorption (VPSA) fractionators and methods.
With the advent of new adsorbent technology for the separation of nitrogen from oxygen, it has proven advantageous to operate air separation plants with a vacuum-pressure swing adsorption (xe2x80x9cVPSAxe2x80x9d) cycle. The vacuum-pressure swing cycle can give better recovery and specific product compared to pressure swing only or vacuum swing only.
U.S. Pat. No. 5,074,892 (xe2x80x9c892xe2x80x9d) discloses a VPSA cycle and compares the VPSA cycle with various parameters and adsorbents to the PSA cycle. It is shown that low pressure ratios in combination with highly lithium exchanged zeolite molecular sieves can yield lower input power and better productivity than other cycle and adsorbent combinations for large industrial systems (15 ton-per-day oxygen production). The definition of pressure ratio is the ratio of the peak adsorption pressure (Ph) to the minimum desorption pressure (Pl). Of course, these pressures must be in terms of absolute pressure.
Despite the very good recovery factors vacuum-pressure swing cycles can achieve, they still require too much input work to compress the feed gas and/or evacuate the waste gas to make them practical in smaller concentrators such as portable oxygen concentrators. For instance, in the case of a small, portable oxygen concentrator for medical use such as that described in U.S. patent application Ser. No. 09/632,099, filed Aug. 3, 2000, which is hereby incorporated by reference as though set forth in full, it is desirable to minimize the power required for compression and evacuation so that the concentrator can have a greater operating duration with a fixed mass of battery pack.
Other VPSA systems such as stationary VPSA systems could also benefit from a development that reduced the power required for compression and evacuation.
U.S. Pat. No. 5,415,683 (xe2x80x9c683xe2x80x9d) describes a VPSA system for the production of oxygen from air including a discrete valving scheme. A sequence of six process steps is claimed including a vent-down (or counter-current depressurization to an intermediate pressure) and a product gas repressurization to an intermediate pressure.
U.S. Pat. No. 3,923,477 (xe2x80x9c477xe2x80x9d) describes a VPSA system including a discrete valving scheme that allows for a vent-down step after a fluid mixture is supplied to an adsorbent bed so as to produce a purified product fluid. In the vent-down step, the valving scheme causes the feed end of the adsorbent bed to communicate with the ambient so that the adsorbent bed is vented down to an intermediate level, atmospheric pressure, between filling the adsorbent bed and evacuating the adsorbent bed. The ""477 patent does not provide for a vent-up step between evacuating the adsorbent bed and filling the adsorbent bed. Instead the ""477 patent provides for back-fill from the product reservoir or from an adjacent bed to raise the pressure in the adsorbent bed to an intermediate level.
U.S. Pat. No. 4,000,990 (xe2x80x9c990xe2x80x9d) describes an apparatus similar to the ""477 patent in that a vent-down step, but no vent-up step, is provided. Instead of a vent-up step, the ""090 patent describes repressurizing the adsorbent bed with product gas.
U.S. Pat. No. 4,065,272 (xe2x80x9c272xe2x80x9d) describes an apparatus similar to the ""477 patent in that a vent-down step, but no vent-up step, is provided. Instead of a vent-up step, the ""272 patent describes back filling the adsorbent bed with product gas.
All of the prior art systems are undesirable for four reasons.
1. These prior art systems all use fluid that has been supplied by the compressor to fill the adsorbent bed. They may use some percentage of product gas for filling the product bed, but this fluid has also been through the compressor and had its pressure raised. With the present invention, it is possible to partially fill the adsorbent bed for xe2x80x9cfreexe2x80x9d with fluid supplied strictly from the ambient and not requiring any compression work.
2. The valving, piping and manifolding of the prior art schemes are more complex and costly.
3. Some of the prior art schemes require valving at the product end of the adsorbent beds that connects to the feed end of the adsorbent beds. This type of valving is prone to leakage of the feed gas into the product gas and thus directly impacts the purity of the product gas.
4. The prior art requires a larger proportion of product gas to back fill the adsorbent bed and hence may limit the recovery or productivity of these systems.
Accordingly, an aspect of the present invention involves a vacuum-pressure swing adsorption process for the separation of components of a fluid mixture where a vent-up step is provided. The vent-up step reduces the amount of feed fluid handled by the compressor, reducing the power consumed in the process. Also, a vent-down step, as known in the prior art, is provided as part of the present invention. The vent-down step reduces the amount of waste fluid handled by the vacuum pump, further reducing the power consumed in the process. An exemplary VPSA process cycle includes (a) transferring a fluid mixture through an adsorbent bed at an elevated pressure Ph so as to produce a purified product fluid; (b) venting the adsorbent bed down to an ambient fluid pressure Pa; (c) applying a vacuum force to the adsorbent bed to bring the adsorbent bed to a reduced pressure Pl; and (d) venting the adsorbent bed up to an ambient fluid pressure Pa.
Another aspect of the present invention involves a vacuum-pressure swing adsorption system for the separation of components of a fluid mixture. The system includes a plurality of adsorbent beds and a valve assembly to provide valving action to transfer a fluid mixture through each adsorbent bed at an elevated pressure Ph so as to produce a purified product fluid, vent the adsorbent bed down to an ambient fluid pressure Pa, cause a vacuum force to be applied to the adsorbent bed to bring the adsorbent bed to a reduced pressure Pl, and vent the adsorbent bed up to an ambient fluid pressure Pa. In a preferred implementation of this aspect of the invention, a rotary valve assembly, especially a face-seal type rotary valve assembly is used. The rotary valve assembly greatly simplifies the routing of fluid flows while packing the fluid distribution function into a very small space. The rotary valve assembly also uses very little power and is known to have very high reliability.
These and other further objects and advantages will be apparent to those skilled in the art in connection with the drawings and the detailed description of the preferred embodiments set forth below.